This disclosure relates to network devices and network communication.
FIG. 1 shows a conventional packet switched network 100—e.g., an Ethernet (IEEE 802.3) network. Packet switched network 100 includes integrated network switches 102 that permit communication of data packets, between network stations (e.g., personal computers, workstations, servers, or other end user devices) (not shown). Network switches 102 can be gateway devices such as, switches, routers, and the like, having network interfaces for forwarding data packets originating from the network stations. Each network station in packet switched network 100 may be associated with a port among network switches 102. Data packets are generally transferred between the network stations through conventional Ethernet media access controller (MAC) circuitry 104 for each port.
Each network switch 102 typically includes a dedicated expansion port 106 that allows each of network switches 102 to be cascaded together through a separate switch network—for example, a crossbar switch 108. Crossbar switch 108 provides an interface to a central processing unit (CPU) 110 (e.g., through a PCI bus 112) for central-management of data packet flow through network switches 102.
The network stations connected to packet switched network 100 are generally grouped into logical workgroups—i.e., virtual local area networks (VLANs). Data packets communicating within a VLAN group require a VLAN tag that identifies the VLAN group, for example, a VLAN type and VLAN ID. Conventionally, a VLAN tag is included as one or more additional fields within a frame header of a given data packet. For example, the Ethernet IEEE 802.3 standard untagged frame format 200 and IEEE 802.3 standard VLAN tagged frame format 202 are shown in FIGS. 2A and 2B, respectively.
Untagged frame format 200 (FIG. 2A) includes a MAC header portion that allocates 6 bytes for a destination address, 6 bytes for a source address, and 2 bytes for length/type data. The destination address specifies either a single recipient (unicast mode), a group of recipients (multicast mode), or a set of all recipients (broadcast mode). Untagged frame format 200 also includes a data portion that is variable in length within a range between 46 and 1500 bytes. A 4-byte frame check sequence (FCS) follows the data portion. In the Ethernet 802.3 protocol, the maximum length for untagged frames is 1518 bytes. VLAN tagged frame format 202 (FIG. 2B) includes a 2 byte VLAN Tag Protocol Identifier (TPID) field and a 2 byte Tag Control Information (TCI) field positioned between the source address field and the length/type field. The TPID field has a fixed, defined value of 8100 in hexadecimal. The first three bits of the TCI field define user priority, allowing 8 priority levels. The fourth bit of the TCI field is the Canonical Format Indicator (CFI), a single-bit flag that is typically set to zero for Ethernet switches. The TCI field also includes a 12-bit VLAN ID (VID) that identifies a particular VLAN—the VID allows for the identification of 4,096 VLANs. In VLAN tagged frame format 202, a 4-byte frame check sequence (FCS) follows the data portion. The frame format for VLAN tagged frames is thus extended in length with respect to untagged frames. In the Ethernet 802.3 protocol, the maximum length for VLAN tagged frames is 1522 bytes.
Referring to FIG. 1, CPU 110 requires switching information, e.g., to maintain central management of data packet flow through packet switched network 100. The switching information can include, for example, an ingress source port and source network switch of a data packet. For a data packet having a VLAN tagged frame passing through a network switch 102, such switching information is typically appended, or pre-appended, to the VLAN tagged frame within a separate field, thus increasing byte size of the data packet.